Field of the Invention
The invention lies in the metallurgy field and pertains, more specifically, to a powder metallurgic method for the production of highly dense shaped parts of an alloy. The alloy contains, besides a portion of at least 20% by weight chromium, iron and, moreover, one of several additional metallic and/or ceramic alloy portions that, in sum, do not amount to more than 10% by weight.
Pure chromium and alloys with high chromium proportions are, if temperatures are low, extremely brittle. It is therefore very difficult to work them with usual powder metallurgic production methods by pressing and sintering the source powder. Therefore, only products or semi-finished products with simple shaping and low density and strength are manufactured of such materials by pressing and sintering, for example, of plate-shaped sputtering targets that are, if necessary, further thermo-mechanically processed for achieving the desired final density and strength.
Iron powder, if compared to chromium powder, is extremely ductile even at low temperatures and can therefore be pressed without problems. Therefore, chromium alloy powders with a sufficiently high iron portion can usually be pressed fairly well. However, it may again be problematic if, besides the iron portion, there are also other alloy portions in smaller quantities in the alloy powder.
Such alloys, especially those with a chromium content of about 90 to 95 weight % an iron portion of about 9 to 4 weight % and a proportion of yttrium or of other rare earths or their oxides to about 1 weight % are used also for interconnectors of solid oxide fuel cells on the basis of their special thermal coefficients of expansion. In order to achieve a sufficiently homogeneous distribution of the present alloy portions that exist in only small quantities of yttrium or rare earths in the powder mixture, a so-called mechanical alloying of the highly clean source powders in high energy mills, normally attritors, is necessary. According to the current state of the art, such alloys are therefore exclusively produced via mechanical alloying.
In doing so, it is disadvantageous that mechanical alloying again leads to an additional strengthening and modification of the morphology of the powders, a fact that strongly reduces the good pressability of the ready-to-press powder. Therefore, complexly formed, highly dense shaped parts made of mechanically alloyed powders, such as interconnectors of fuel cells, cannot be produced any longer by pressing and sintering in a form near to the final shape.
Therefore, for the production of such parts, firstly it is necessary to manufacture raw parts as simply formed elements with sufficient oversize, or as semi-finished product, by pressing and sintering, thus reaching a maximum density of about 70%. In order to attain the necessary final density for a sufficient mechanical strength, these raw parts must be further deformed by mechanical reshaping, for example, by rolling. The desired final shapes that in case of interconnectors may be, for example, channels and surfaces structured by naps, must then be mechanically and/or electrochemically worked out of these raw parts with high expenditure in money and time.
It is true that, with other powder metallurgic production methods, for example, metal injection molding, it is possible to produce complexly formed shaped parts in a form near to the final shape. However, it is disadvantageous that such methods require high portions of binding agents that cause a shrinkage of the shaped parts during the sintering of about up to 10% and more, a fact that again has a disadvantageous effect on the dimensional stability and the faithfulness regarding the shape of the finished part. Moreover, according to the contemporary state of the art, the technology of the metal injection molding is only limited to small parts.